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KOH (Potassium Hydroxide) Preparation for Fungal Nail Infections

Exam Overview

It is not always possible to accurately
diagnose a
fungal nail infection based on appearance alone. A
potassium hydroxide (KOH) preparation test can help determine whether you have
a fungal nail infection.

Your doctor will collect a sample of skin
and/or nail fragments (debris) under the infected nail. If a sample of debris
can't be removed, he or she can take a nail sample by lightly scraping the
nail near the infected area or by using a small blade to shave off a piece of
nail.

Nail or debris samples are placed on a slide with a
potassium hydroxide (KOH) solution and gently heated. The solution slowly
dissolves the skin and nail cells, leaving the fungus cells. The fungus cells
can then be seen with a microscope. Color stains can be used to make the fungi
easier to see.

Why It Is Done

If you have discolored, damaged,
broken, or thickened nails or have a buildup of skin and nail fragments
(debris) under a nail, a KOH test can help confirm whether a fungal nail
infection is present.

Results

Normal

No fungi are present in the debris or nail
sample. But the test may be done again using more samples taken from other
parts of the infected nail. This is because there may no longer be fungi on the
edge of the infected area, where it is easiest to scrape for a sample, or in
the dead tissue, even if you have a fungal nail infection.

Abnormal

Fungi are present in the debris or nail
sample.

What To Think About

Not all nail diseases
are fungal nail infections. Because of this and the
risks linked with oral antifungal medicine, your
doctor will probably confirm that you have a fungal infection using a KOH
preparation test if you are considering taking antifungal pills (oral
treatment). Many insurance companies now require testing to verify a fungal
infection before paying for medicines.

If your test is normal, your doctor may
do other tests to determine the cause of the nail damage.

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